This invention is directed to the salinity measurement of sea water, and in particular to a conductivity cell for the determination of sea water salinity.
For a long time there has been a requirement for a moorable sensor chain to record conductivity and temperature values in coastal regions from which salinity may be determined. A major limitation in past attempts to fill this need has been lack of stability in the conductivity sensor cell constant caused by biological activity, dimensional changes, or corrosion. Another problem in chain construction was that most conductivity sensors employed an inductive principle which made it difficult to place them more than a few feet from their associated electronics. Thus a chain would be assembled from a number of individual sensors complete with electronics rather than permitting them to be multiplexed onto a single electronics board, a much cheaper construction. In contrast the four electrode method of measuring conductivity typically operates at frequencies where the inductive effects associated with long lines separating sensor and electronics no longer are significant.
By modifying the electrodes, it is possible to minimize the consequences of corrosion or dimensional changes in the immediate vicinity of the electrodes themselves. In the four electrode system described in the publication by T. M. Dauphinee--"Some Applications of DC and Square Wave AC Techniques to Undersea Measurements"--Paper No. 68-635, Instrument Society of America Annual Conference, 1968, two "voltage" electrodes sense a potential difference between fixed points along a current path between two "current" electrodes, the current being varied to keep this voltage constant. The current then provides a measure of the cell conductance. Changes in current flow path due to corrosion, etc., will effect the sensed voltage particularly if the electrodes carry any current due either to the finite input impedance of the amplifier or because they partially shunt the seawater circuit. A very high input impedance is offered by Dauphinee's circuit and the voltage electrodes are recessed to stay out of the current path. Nevertheless, variations in current density over the current electrodes as a result of changing metal/sea water interface conditions can alter the sensed voltage difference. A development of this system used for the Guildline Bench salinometer is described in the publication by T. M. Dauphinee and H. P. Klein, "A New Automated Laboratory Salinometer", Sea Technology, 16, 1975, pages 23-25. All four electrodes are placed in side arms away from the main current path so that current flow along the path interval containing the voltage electrodes is almost independent of local changes at the electrode surface. Another factor has been the need to obtain a high enough cell impedance in typical sea water to allow use of easily obtainable electronic circuit components within their normal operating range. This has usually required considerable physical constrictions in current flow which in turn have created a problem in cell flushing as different water masses move past the sensor in the chain.